1. Field of the Invention
The present invention relates to a group III-V nitride series semiconductor substrate and its assessment method. In particular, it relates to a group III-V nitride series semiconductor substrate which allows a compound semiconductor layer to be grown on the substrate in such a way that it is flat and that impurities are uniformly distributed, and to an assessment method which allows quick and convenient assessment as to whether a substrate allows a compound semiconductor layer to be grown on the substrate in such a way that it is flat and that impurities are uniformly distributed.
2. Description of the Related Art
The application of nitride series semiconductor materials to short-wavelength light-emitting devices has been actively made because of wide bandgaps and direct transition type band-to-band transitions. Nitride semiconductor series devices are obtained by performing epitaxial growth on a base substrate, using a vapor phase growth method such as metal organic vapor phase epitaxy (MOVPE), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), or the like. In a crystal obtained by these growth methods, however, there exist many crystalline defects. The reason for that is because there is no base substrate of a different kind that matches the lattice constant of a nitride semiconductor. For this reason, a self-standing substrate of the same kind (e.g., GaN self-standing substrate) that matches the lattice constant of a nitride semiconductor has been demanded.
As a crystalline defect reducing technique in a GaN epitaxial growth method, an ELO (epitaxial lateral overgrowth) method has been known (see e.g., Japanese patent application laid-open No.11-251253). ELO is a technique for obtaining a low-dislocation GaN layer by fabricating a mask with stripe openings in abase substrate, and selectively growing GaN initial growth nuclei in the openings. With ELO, a GaN layer is formed in a base substrate, followed by removal of the base substrate, so that a good-quality GaN self-standing substrate is obtained.
Even in the GaN self-standing substrate obtained by the above-mentioned method, however, there have remained the problems with surface flatness of the GaN self-standing substrate, impurity distribution, and warpage of the substrate, etc. For instance, in case of poor surface flatness of the GaN self-standing substrate, poor impurity distribution, or large warpage of the substrate, even if an epitaxial layer for light-emitting devices is grown on that GaN self-standing substrate with MOVPE, the surface flatness of the growth layer and impurity distribution become poor. For this reason, when it is cut into a plurality of chips to fabricate a plurality of light-emitting devices, the light-emission intensity of the individual light-emitting devices is varied widely despite their being formed from the same GaN self-standing substrate. Also, it is difficult to apply lithography to such a substrate, which has a significant effect on good-quality product yield of the devices.